Facsimile (or FAX) transmission systems are well known in the prior art for point-to-point transmission of pictorial information. Facsimile transmission is particularly widely used for transmitting messages which are in a time-stationary graphic form, such as writing, typewriting, charts, graphs, pictures, photographs, etc. The transmission system may be either analog or digital and may employ any convenient modulation technique. In the past, commercial facsimile systems have employed primarily analog amplitude or frequency modulation, but digital systems may become more common in the future.
Often, it is desirable to insure the privacy of a communication between the point of origin and the intended destination, such that the transmission over an unsecure channel will be unintelligible to third parties who intentionally or unintentionally intercept the transmission.
In particular, various types of privacy systems already are well known for rendering facsimile transmissions unintelligible for transmission over an exposed transmission link, to make difficult or impossible reconstruction of the message content by unauthorized receivers.
Often, however, in facsimile systems graphic information to be transmitted may include one or more straight lines perpendicular to the direction of scanning of the input medium, or nearly so. An example would be plots of well logs showing the results of geological surveying and exploration for gas and oil wells. Such a line (or lines) may or may not be a significant portion of the message. More importantly, though, if not scrambled effectively it (or they) may provide an obvious clue to the scrambling algorithm, thereby permitting the unauthorized recipient to figure out how to decode the transmission.
In a typical facsimile transmission system, the image to be transmitted is placed on a transparent, rotating drum and a stationary optical sensor "reads" the image. The sensor is advanced in a direction parallel to the axis of drum rotation, at a rate much slower than the rotation rate of the drum, to do a line-by-line scan of the image. Thus, if the image contains a straight line perpendicular to the direction of scanning, as the drum rotates the sensor periodically will detect the line. Such detection of the line by the sensor will cause an abrupt change in the "clear" (i.e., unscrambled) facsimile signal generated from the sensor's output. If the scrambling algorithm transforms this abrupt change, in real time, into any abrupt change in the scrambled facsimile signal, the periodicity of the sampling is revealed thereby; in turn, the periodicity reveals both the scanning rate and the presence and orientation of the line. This information may permit the eventual unscrambling of the entire message.
Consequently, in any system of this type, designed for the private transmission of facsimile messages, it is essential to provide a scrambling technique which can hide the presence of a line perpendicular to the direction of scan.
There appear to be basically two ways to hide the presence of such a simple straight line. In the first method, the repetitive nature of the scanner signal is hidden by moving the abrupt transition in the scrambled signal to a different time during the scan period for each line in the scan--i.e., randomizing the time of occurrence of the abrupt transition. In the second method, the fact of the existence of an abrupt transition is disguised by transmitting a scrambled signal that is not just a one-for-one transformation of the instantaneous value of the clear signal. The former approach is used in the present invention; additionally, the latter technique may be combined therewith, if the added security is desired and the additional complexity can be tolerated.